In recent years, a requirement for decreasing electric power consumption is increased for the above hydraulic control device, in order to make a contribution to decrease a fuel consumption ratio of the vehicle. The present disclosure focused on the electric power consumption in a hydraulic-pressure control unit among the electric power consumption of the hydraulic control device.
The hydraulic-pressure control unit has a solenoid and a valve portion, wherein the solenoid is electrically controlled in accordance with a command value for a hydraulic control pressure and an opening degree of the valve portion is adjusted by an electric power supply amount to the solenoid. The hydraulic-pressure control unit, which is provided for each of the friction elements, adjusts a hydraulic pressure produced at a hydraulic-pressure producing unit in order to supply such adjusted hydraulic pressure to the friction element as the hydraulic control pressure.
When a gear-change demand is generated, friction plates of the respective friction elements related to such a gear change are engaged with each other or dis-engaged from each other. Therefore, the command value for the hydraulic control pressure is changed with time for each of the hydraulic-pressure control units related to the gear change. Namely, an electric power supply amount to the solenoid of the hydraulic-pressure control unit is changed with time in order to adjust the hydraulic control pressure. Thus, the electric power is consumed. In each of the friction elements, a coupling condition of the friction plates is changed from an engaged condition to a dis-engaged condition, or vice versa, through a transient state between the engaged and dis-engaged conditions.
When the vehicle runs at a constant speed, for example, at a high speed, the gear-change demand is not generated, so that the engaged condition and the dis-engaged condition of the respective friction elements are maintained. However, the electric power is consumed in each of the solenoids of the hydraulic-pressure control units, even when each of the friction elements is in its steady state, in which the engaged-condition or the dis-engaged condition for the friction plates is maintained.
Each of the hydraulic-pressure control units is composed of either a normally low-pressure type valve (an N/L type valve) which outputs a low pressure when no electric power is supplied to the solenoid of the hydraulic-pressure control unit or a normally high-pressure type valve (an N/H type valve) which outputs a high pressure when no electric power is supplied to the solenoid of the hydraulic-pressure control unit.
In a case that the hydraulic-pressure control unit is composed of the N/L type valve, it is necessary to maintain the electric power supply to the solenoid in order to keep the opening degree of the valve portion at a high-pressure side, when the engaged condition of the friction element connected to the hydraulic-pressure control unit of the N/L type is maintained. Therefore, the electric power is continuously consumed.
On the other hand, in a case that the hydraulic-pressure control unit is composed of the N/H type valve, it is necessary to maintain the electric power supply to the solenoid in order to keep the opening degree of the valve portion at a low-pressure side, when the dis-engaged condition of the friction element connected to the hydraulic-pressure control unit of the N/H type is maintained. Therefore, the electric power is likewise consumed.
The inventors of the present disclosure have conducted the study of reducing the electric power consumption, which is generated in the hydraulic-pressure control unit even when the friction elements are in the steady state.
Japanese Patent Publication No. 2010-084855 (hereinafter, a first prior art) discloses a hydraulic control device for an automatic transmission apparatus, according to which all of hydraulic-pressure control units are composed of N/L type valves in order to reduce electric power consumption in a specific steady state of friction elements. Another Japanese Patent Publication No. 2007-139181 (hereinafter, a second prior art) also discloses a hydraulic control device for the automatic transmission apparatus, according to which a hydraulic passage is provided for introducing hydraulic pressure into an axial end portion of a valve member of a hydraulic-pressure control unit and an ON-OFF operation of the hydraulic passage is controlled separately from a solenoid of the hydraulic-pressure control unit. A valve portion of the second prior art is maintained at its high-pressure side or its low-pressure side, without supplying electric power to the solenoid, so that the electric power consumption is not generated in the solenoid.
However, according to a structure of the above first or the second prior art, it is possible to avoid the generation of the electric power consumption in the solenoid, only when the valve portion is maintained either at the high-pressure side or at the low-pressure side. Therefore, the electric power consumption is unavoidable when the valve portion is maintained at the other side of the high-pressure side or the low-pressure side.
Accordingly, the structure of the above first and the second prior arts is not sufficient to satisfy an object, which the present inventors have aimed at, that is, the object to reduce the electric power consumption generated in the hydraulic-pressure control units when the friction elements are in the steady state.